1. Field of the Invention
The present invention relates to a fiber-optic sensor with a protective tube and method for installing a fiber-optic sensor in the protective tube, where the fiber-optic sensor is pushed into the protective tube through an end of the protective tube open during installation.
2. Description of the Related Art
For applications in process measurement technology and process engineering, multi-point thermometers, in which a number of temperature measuring points are disposed along the same fiber of a fiber-optic sensor and which make it possible to simultaneously measure a number of temperatures, are frequently of interest. Here, only one fiber-optic sensor is needed for almost any given number of measuring points, with a connector, to which a transmitter is connected, which is used to determine the individual measured temperature values through optoelectronic signal processing. For example, within a process engineering system, measured temperature values established in this way can be forwarded with the aid of the transmitter to a superordinate control or a superordinate command and control system, and can be used for closed-loop control of a process running on the system. Temperature measurements of this type are not only needed, for example, at reactors, pipes, vessels, tanks but also in drilled holes, tunnels or culverts. DE 10 2004 031 324 A1, discloses a conventional fiber-optic sensor that is suitable for measuring the temperature in a process engineering system.
In fiber-optic sensors with what are known as “Bragg gratings” for detecting the temperatures at different points of the fiber, the Bragg grating is effectively “burnt-in” by optical illumination at the different measuring points. Each sensor element then measures “its own” local temperature, in that each sensor reflects, as a function of the temperature, a wavelength of the light radiated in through the fiber from the transmitter. In the transmitter, the wavelength of the reflected light is determined precisely and is computed back to the associated temperature values via a calibration equation. The fact that each sensor element is burnt in for a different wavelength means that, in principle, the transmitter can read out any given number of sensor elements. The number of sensor elements on a fiber-optic sensor frequently lies between 5 and 30.
To protect against mechanical stress, the actual measuring fiber is usually enclosed in a stainless steel hollow tube, the internal diameter of which amounts to, e.g., 0.7 mm, and the external diameter of which amounts to, e.g., 1.2 mm. In order to damp out any vibrations that might occur at the installation site, a woven protective sleeve can be pulled over the stainless steel tube. In order to simplify matters here, a measuring fiber with a stainless steel hollow tube and also a measuring fiber with a stainless steel hollow tube and protective sleeve are uniformly referred to as a fiber-optic sensor.
At the respective installation site, fiber-optic sensors are usually mounted in a permanently-installed protective tube, which is located, for example, on or in a chemical reactor. The fiber-optic sensor thus must be installed in the protective tube on site. EP 2 202 552 A1 discloses a suitable method for installing a fiber-optic sensor in a protective tube. Here, in order to install it, the fiber-optic sensor is pushed through an open end of the protective tube into the tube and brought into its axial target position, so that the individual sensor elements can detect temperatures at the desired points along the axis of the protective tube.
A protective tube, which is required for the mechanical protection of a fiber-optic sensor, can be problematic during the measurement of variable temperatures, however, because the different layers that are disposed around the actual measurement fiber make it more difficult to rapidly detect temperature changes in the process. The flow of heat from the process medium to the measurement fiber is especially slowed down by the air space, which may be present within the protective tube, which exists between the fiber-optic sensor and the inner wall of the protective tube if the sensor is not resting against the inner wall. An advantage of fiber-optic sensors for temperature measurements, i.e., their potentially fast reaction time because of the low thermal capacity of the sensor, can therefore disadvantageously be lost as a result of installing the sensor in a protective tube. It is known that the reaction time of a fiber-optic sensor, which amounts to 3 seconds without a protective tube, for example, can increase by a factor of 20 when the same sensor is installed in a protective tube with a diameter of 12 mm. This represents a significant disadvantage with respect to measurement accuracy and dynamics.